# student_management_system.py

class Student:
    def __init__(self, name, age):
        self.name = name
        self.age = age
        self.grades = {}

    def add_grade(self, subject, grade):
        self.grades[subject] = grade

    def get_average_grade(self):
        if not self.grades:
            return 0
        total = sum(self.grades.values())
        return total / len(self.grades)

# 测试
student = Student("张三", 20)
student.add_grade("数学", 90)
student.add_grade("英语", 85)
print(f"{student.name}的平均成绩：{student.get_average_grade()}")